In my exerience, having each character research individually has worked the best. You can group them together increasing the chance of a successful research mission, but it also slows down the amount of attempts. Rebel characters have about a 50% success rate and the same goes for the Imperials except Bevel, who gets a higher success chance. Diplomacy works the same way. Take a large group of characters with bad diplomacy and group them together. Each diplomatic mission attempt will get a hidden roll to see if a character is successful. If the attempt is successful, there is a change in loyalty for the system, if not, you get a failed attempt message and the diplomat will continue the mission until you tell them to stop or continue till the system is 100%. When you group a few characters together, each character gets a hidden roll. If one is successful, the diplomacy mission is a success and the diplomacy for the system is adjusted and the character who got the successful roll will get a diplomacy adjustment to their permanent statistics. If a research mission is successful, there is not an adjustment for the character. Mind you, this is all based on what I have experienced with the game. I have not delved into the code side of the game, so this is based on opinion and not fact, so take it for what it is worth.